If using precooked, frozen chicken breasts reheat according to directions on the package for a quick, easy meal.
Alternatively, you can cook your own. First pound chicken breasts to 1/2-inch thickness. Season to taste and cook on an electric grill. Or marinate chicken overnight using your marinade of choice. Depending on the size of your chicken breasts, you may need to cut them in half to fit the bun or croissant.
If using ciabatta rolls, toast under a broiler until golden. To assemble sandwich, spread mayonnaise on ciabatta roll or croissant. Then top with hot chicken breast, Swiss cheese and avocado. Serve immediately.